Synthesis and characterization of β-cyclodextrin inclusion complexes of thymol and thyme oil for antimicrobial delivery applications

Essential Oils-Loaded Electrospun Biopolymers: A Future Perspective for Active Food Packaging

The growth of global food demand combined with the increased appeal to access different foods from every corner of the globe is forcing the food industry to look for alternative technologies to increase the shelf life. Essential oils (EOs) as naturally occurring functional ingredients have shown great prospects in active food packaging. EOs can inhibit the growth of superficial food pathogens, modify nutritious values without affecting the sensory qualities of food, and prolong the shelf life when used in food packaging as an active ingredient. Since 2016, various reports have demonstrated that combinations of electrospun fibers and encapsulated EOs could offer promising results when used as food packaging. Such electrospun platforms have encapsulated either pure EOs or their complexation with other antibacterial agents to prolong the shelf life of food products through sustained release of active ingredients. This paper presents a comprehensive review of the essential oil-loaded electrospun fibers that have been applied as active food packaging material.

Preparation and Characterization of Chitosan–Alginate Polyelectrolyte Complexes Loaded with Antibacterial Thyme Oil Nanoemulsions

Biomedical industries are attempting to utilize natural materials, as they are bio-compatible, non-toxic, and show bioactive properties, like antimicrobial activity. In this study, natural polyelectrolyte complexed chitosan/alginate films (PECs) were prepared via a casting/solvent evaporation technique, and their characteristics and drug release properties were investigated. PEC films made with two different overall polymer contents, 0.4 and 1 w/v%, were loaded with thyme oil nanoemulsion as drug carrier. The structure of the films was studied by FTIR and optical and scanning electron microscopy. Prepared PEC films had good mechanical and water vapor permeability properties. Release of the thyme oil from the pH-sensitive PEC films (TM-PEC) was detected and followed by UV spectroscopy. The results indicated that the drug release rate of TM-PEC films was the fastest when the chitosan content was 1 %w/v, and various mathematical models were analyzed for investigating the drug release mechanism. Antibacterial tests were performed by counting the number of surviving gram-negative and gram-positive bacteria. The in vitro test indicated the limitation Escherichia coli (E. coli) and Staphylococcus aureus (S.aureus) growth in the presence of TM-PEC films. The MTT test showed more cell viability of the TM-PEC film in comparison with that of the PEC film without TM. Based on the measured physical and antibacterial properties, the chitosan–alginate PEC films loaded with antibacterial essential oils can be considered for biomedical applications, such as wound dressings or controlled release systems.

Inclusion Complexes of Lycopene and β-Cyclodextrin: Preparation, Characterization, Stability and Antioxidant Activity

In this study, the inclusion complexes of lycopene with β-cyclodextrin (β-CD) were prepared by the precipitation method. Then the inclusion complexes were characterized by the scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV), microscopic observation, liquid chromatography, differential scanning calorimetry (DSC) and phase-solubility study. Moreover, the stability and antioxidant activity were tested. The results showed that lycopene was embedded into the cavity of β-CD with a 1:1 stoichiometry. Moreover, the thermal and irradiant stabilities of lycopene were all significantly increased by the formation of lycopene/β-CD inclusion complexes. Antioxidant properties of lycopene and its inclusion complexes were evaluated on the basis of measuring the scavenging activity for 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydroxyl and superoxide anion radicals. The results showed that the scavenging activity of DPPH radicals was obviously increased by the formation of the inclusion complex with β-cyclodextrin at concentrations of 5–30 μg/mL, however, some significant positive effects on the scavenging activity of hydroxyl and superoxide anion radicals were not observed and the reasons are worth further study.

Encapsulation of Essential Oils for the Development of Biosourced Pesticides with Controlled Release: A Review

Essential oil (EO) encapsulation can be carried out via a multitude of techniques, depending on applications. Because of EOs’ biological activities, the development of biosourced pesticides with EO encapsulation is of great interest. A lot of methods have been developed; they are presented in this review, together with the properties of the final products. Encapsulation conserves and protects EOs from outside aggression, but also allows for controlled release, which is useful for applications in agronomy. The focus is on the matrices that are of interest for the controlled release of their content, namely: alginate, chitosan, and cyclodextrin. Those three matrices are used with several methods in order to create EO encapsulation with different structures, capacities, and release profiles.

Browning control of fresh-cut Chinese yam by edible coatings enriched with an inclusion complex containing star anise essential oil

Star anise essential oil (SAEO) has a variety of antioxygenic and antimicrobial properties, and is widely used in food preservation. However, its application is still challenging due to poor water solubility and physicochemical stability. We now report that encapsulation of SAEO in hydroxypropyl-β-cyclodextrin (HPCD) enhances its water solubility, as well as its thermal, storage, and photostability. The solubility of SAEO encapsulated by HPCD was increased by 47.5 times at 45 °C, and the onset temperature of thermal volatilization was delayed by at least 200 °C. The encapsulated material is also more uniformly and more stably dispersed in xanthan gum, and is thus released in a controlled manner. Importantly, fresh-cut Chinese yam coated with xanthan gum containing encapsulated SAEO is more effectively preserved, as assessed using weight loss, L* value, browning index, and polyphenol oxidase activity. The data suggest that the encapsulated SAEO reduced the weight loss of the samples by more than 30%, and the encapsulation of HPCD increased the inhibitory effect of SAEO on browning and polyphenol oxidase activity of the samples by nearly 8 times and more than 7 times, respectively. Collectively, SAEO encapsulated in HPCD is promising as a preservative coat for fresh-cut fruits and vegetables.

The encapsulation of hydroxypropyl-β-cyclodextrin improved the inhibitory effect of star anise essential oil on the browning of fresh-cut Chinese yam.

Characterization of Cyclodextrin/Volatile Inclusion Complexes: A Review

Cyclodextrins (CDs) are a family of cyclic oligosaccharides that constitute one of the most widely used molecular hosts in supramolecular chemistry. Encapsulation in the hydrophobic cavity of CDs positively affects the physical and chemical characteristics of the guests upon the formation of inclusion complexes. Such a property is interestingly employed to retain volatile guests and reduce their volatility. Within this scope, the starting crucial point for a suitable and careful characterization of an inclusion complex is to assess the value of the formation constant (K_f), also called stability or binding constant. This task requires the application of the appropriate analytical method and technique. Thus, the aim of the present paper is to give a general overview of the main analytical tools used for the determination of K_f values for CD/volatile inclusion complexes. This review emphasizes on the advantages, inconvenients and limits of each applied method. A special attention is also dedicated to the improvement of the current methods and to the development of new techniques. Further, the applicability of each technique is illustrated by a summary of data obtained from the literature.

Effect of Selective Encapsulation of Hydroxypropyl-β-cyclodextrin on Components and Antibacterial Properties of Star Anise Essential Oil

Star anise essential oil (SAEO) is a plant essential oil with good antibacterial activity, but its applications are limited due to its high volatility, strong smell, and unstable physical and chemical properties. The effect of selective encapsulation of SAEO by hydroxypropyl-β-cyclodextrin (HPCD) on its compositions, volatility stability and antibacterial activity was investigated. The GC-MS results indicated that the compositions reduced and content of the compositions of SAEO changed after encapsulation. Most of the components in SAEO were successfully encapsulated by HPCD, which can be supported by data from FTIR and ¹H NMR. According to the molecular modeling results, the three guest molecules (trans-anethole, estragole and trans-foeniculin) were all docked in the cavity of HPCD on the isoallyl (or allyl) side. The volatile stability of SAEO before and after encapsulation was evaluated by electronic nose, and the results confirmed that encapsulation significantly reduced the irritating smell of SAEO and makes the clathrate have a sustained release effect. Furthermore, in the antibacterial test, the selective encapsulation of HPCD improved the inhibition effect of SAEO on Rhizopus stolonoifer, Saccharomyces cerevisiae, and E. coli and its antibacterial stability in 24 h.

Supercritical CO2 impregnation of PLA/PCL films with natural substances for bacterial growth control in food packaging

Biodegradable polymers with antibacterial properties are highly desirable materials for active food packaging applications. Thymol, a dietary monoterpene phenol with a strong antibacterial activity is abundant in plants belonging to the genus Thymus. This study presents two approaches for supercritical CO₂ impregnation of poly(lactic acid)(PLA)/poly(ε-caprolactone)(PCL) blended films to induce antibacterial properties of the material: (i) a batch impregnation process for loading pure thymol, and (ii) an integrated supercritical extraction-impregnation process for isolation of thyme extract and its incorporation into the films, operated in both batch or semi-continuous modes with supercritical solution circulation. The PCL content in films, impregnation time and CO₂ flow regime were varied to maximize loading of the films with thymol or thyme extract with preserving films' structure and thermal stability. Representative film samples impregnated with thymol and thyme extract were tested against Gram (-) (Escherichia coli) and Gram(+) (Bacillus subtilis) model strains, by measuring their metabolic activity and re-cultivation after exposure to the films. The film containing thymol (35.8 wt%) showed a strong antibacterial activity leading to a total reduction of bacterial cell viability. Proposed processes enable fast, controlled and organic solvent-free fabrication of the PLA/PCL films containing natural antibacterial substances at moderately low temperature, with a compact structure and a good thermal stability, for potential use as active food packaging materials.

Thymol/cyclodextrin inclusion complex nanofibrous webs: Enhanced water solubility, high thermal stability and antioxidant property of thymol

The development of novel nanomaterials that provide an efficient encapsulation and protection for the active food additives is one of the main focuses of current research efforts at food application areas. From this point of view, in this study, nanofibrous webs from inclusion complexes (IC) of modified cyclodextrins (hydroxypropyl-β-cyclodextrin (HPβCD), hydroxypropyl-γ-cyclodextrin (HPγCD) and methyl-β-cyclodextrin (MβCD)) and essential oils compound (i.e. thymol) was produced through electrospinning technique. While pure thymol has a highly volatile nature, the volatility of thymol was effectively suppressed by the inclusion complexation and ~88-100% (w/w) of thymol was preserved in electrospun thymol/cyclodextrin inclusion complex nanofibers (Thymol/CD-IC NF). The aqueous solubility enhancement for hydrophobic thymol was demonstrated by phase solubility diagram which also suggested the 1:1M inclusion complexation between thymol and CD molecules. Besides, Thymol/CD-IC NF displayed quite fast disintegration in water compared to poorly water soluble thymol. By inclusion complexation, high temperature stability for volatile thymol was achieved for Thymol/CD-IC NF samples. The loading of thymol in Thymol/CD-IC NF conferred DPPH radical scavenging ability to these nanofibrous webs. So, the Thymol/CD-IC NF have shown antioxidant activity along with enhanced water solubility and high thermal stability of thymol. In brief, encapsulation of essential oil compounds such as thymol in electrospun CD-IC nanofibers can promote its potential application in food and oral-care products by associating the large surface area of nanofibrous webs along with CD inclusion complexation which provides enhanced water solubility and antioxidant property, and high temperature stability for thymol.

Optimisation of β-cyclodextrin inclusion complexes with natural antimicrobial agents: thymol, carvacrol and linalool

Beta-cyclodextrin (β-CD) inclusion complexes with naturally derived antimicrobial (AM) agents: thymol, carvacrol and linalool were prepared using a co-precipitation technique. Conditions including solvent composition, temperature, reaction time and total solvent volume were investigated to optimise the inclusion efficiency (IE) and yield. Electrospray ionisation mass spectrometry was used to confirm the formation of the thymol/β-CD complex and gas chromatography was used to quantify the amount AM agent that was encapsulated, absorbed onto the surface, or remaining in the filtered solvent. The systematic optimisation of the conditions improved both the yield of the complex and the IE of the AM agents compared to previously reported methods that have been applied to other agents. Using a 1:1 mole ratio of the AM agent to β-CD, the optimised parameters resulted in maximum yields of 87, 84 and 86% (w/w) for thymol, carvacrol and linalool, respectively, with IE's close to 100% (w/w) for each agent.

A review on influencing factors on the minimum inhibitory concentration of essential oils

With growing interest in essential oils as natural preservatives in the food industry, the literature is expanding enormously. To understand the antimicrobial activity of essential oils, the antimicrobial mechanism of individual essential oil (EO) compounds, and their minimum inhibitory concentrations (MICs), are interesting starting points for research. Therefore, and to get insight into the factors influencing their antimicrobial activities, the Web of Science was searched for MICs of EO compounds (1995-2016). Many MICs for individual EO compounds have already been reported in the literature, but there is large variability in these data, even for the MIC of the same compound against the same species. No correlation was found between the tested structural parameters of EO compounds (polarity, water solubility, dissociation constant, molecular weight and molecular complexity) and their MICs against all microorganisms, Gram-negative bacteria, Gram-positive bacteria and fungi. Few clear differences in sensitivity between microorganisms could be found. Based on this review it is clear that different incubation conditions, culture media and the use of emulsifiers/solvents have an influence on the MIC, causing big variance. This review points out the need for a good international standard method to assess the antimicrobial activity of EO compounds for better comparability between studies.

Antibacterial poly(ethylene oxide) electrospun nanofibers containing cinnamon essential oil/beta-cyclodextrin proteoliposomes

A novel antibacterial packaging material was engineered by incorporating cinnamon essential oil/β-cyclodextrin (CEO/β-CD) proteoliposomes into poly(ethylene oxide) (PEO) nanofibers by electrospinning technique. Herein, PEO was a stabilizing polymer and used as electrospinning polymeric matrix for the fabrication of CEO/β-CD proteoliposomes nanofibers. The nanoliposomes were inlaid with protein are defined as proteoliposomes. Taking advantage of bacterial protease secreted from Bacillus cereus (B. cereus), the controlled release of CEO from proteoliposomes was achieved via proteolysis of protein in proteoliposomes. The CEO/β-CD inclusion complex was prepared by the aqueous solution method and characterized by Raman and FTIR spectroscopy. After the treatment of CEO/β-CD proteoliposomes nanofibers packaging, the satisfactory antibacterial efficiency against B. cereus on beef was realized without any impact on sensory quality of beef. This study demonstrated that the CEO/β-CD proteoliposomes nanofibers can significantly extend the shelf life of beef and have potential application in active food packaging.

Antibacterial electrospun zein nanofibrous web encapsulating thymol/cyclodextrin-inclusion complex for food packaging

Thymol (THY)/γ-Cyclodextrin(γ-CD) inclusion complex (IC) encapsulated electrospun zein nanofibrous webs (zein-THY/γ-CD-IC-NF) were fabricated as a food packaging material. The formation of THY/γ-CD-IC (1:1 and 2:1) was proved by experimental (X-ray diffraction (XRD), thermal gravimetric analysis (TGA), ¹H NMR) and computational techniques. THY/γ-CD-IC (2:1) exhibited higher preservation rate and stability than THY/γ-CD-IC (1:1). It is worth mentioning that zein-THY/γ-CD-IC-NF (2:1) preserved much more THY as observed in TGA and stability of THY/γ-CD-IC (2:1) was higher, as shown by a modelling study. Therefore, much more THY was released from zein-THY/γ-CD-IC-NF (2:1) than zein-THY-NF and zein-THY/γ-CD-IC-NF (1:1). Similarly, antibacterial activity of zein-THY/γ-CD-IC-NF (2:1) was higher than zein-THY-NF and zein-THY/γ-CD-IC-NF (1:1). It was demonstrated that zein-THY/γ-CD-IC-NF (2:1) was most effective in inhibiting the growth of bacteria on meat samples. These webs show potential application as an antibacterial food packaging material.

Development of Water-Triggered Chitosan Film Containing Glucamylase for Sustained Release of Resveratrol

There is a paradox when incorporating enzyme into an edible chitosan film that chitosan is dissolved in acid solution and enzyme activity is maintained under mild conditions. A method for maintaining the pH of the chitosan solution at 4-6 to prepare a chitosan film containing β-cyclodextrin, resveratrol-β-cyclodextrin inclusion (RCI), was developed, using glucamylase and acetic acid. A considerable amount of resveratrol was released by the glucamylase-incorporated film within 15 days, and the maximum amount released was 46% of the total resveratrol content. The highest resveratrol release ratio (released resveratrol/total resveratrol) was obtained in the film with 6 mL of RCI. Scratches and spores were generated on the surface of the glucamylase-added film immersed in water (GAFW) for 7 days because of β-cyclodextrin hydrolysis during film drying and water immersion. RCI and β-cyclodextrin were extruded from the film surface and formed teardrops, which were erased by water on the GAFW surface but appeared on the glucamylase-added film without water immersion (GAF). The bubbles generated by the reaction of acetic acid and residual sodium bicarbonate were observed in both glucamylase-free films immersed in water (GFFW) for 7 days and without water immersion (GFF). The FT-IR spectra illustrated that the covalent bond was not generated during water immersion and β-cyclodextrin hydrolysis. The crystal structure of chitosan was destroyed by water immersion and β-cyclodextrin hydrolysis, resulting in the lowest chitosan crystallization peak at 22°. The increasing of water holding capacity determined by EDX presented the following order: GAF, GFFW, GFF, and GAFW.

Application of cellulose acetate for controlled release of thymol

Cellulose acetate (CA) was investigated as a carrier towards development of material with controlled release of thymol as a natural substance with strong antibacterial properties using high pressure techniques. Effect of thymol content on CA was confirmed by SEM, FTIR and DSC methods. Kinetic of thymol release from CA was tested using simulated gastric and intestinal fluids (hydrochloric acid and phosphate buffer saline). Results were correlated with Korsmeyer-Peppas and Weibull model. Depending on the thymol content and chemical nature of the release medium, the time of thymol release varied from one to three days indicating CA as a promising carrier of thymol with potential uses from medicine to agriculture. The impregnated CA showed antibacterial activity against 23 tested bacterial strains including methicillin-resistant Staphylococcus aureus (MRSA) which is particularly important bearing in mind that this strain causes fatal infections in humans and animals.

Determination of formation constants and structural characterization of cyclodextrin inclusion complexes with two phenolic isomers: carvacrol and thymol

Carvacrol and thymol have been widely studied for their ability to control food spoilage and to extend shelf-life of food products due to their antimicrobial and antioxidant activities. However, they suffer from poor aqueous solubility and pronounced flavoring ability that limit their application in food systems. These drawbacks could be surpassed by encapsulation in cyclodextrins (CDs). Applications of their inclusion complexes with CDs were reported without investigating the inclusion phenomenon in deep. In this study, inclusion complexes were characterized in terms of formation constants (K_f), complexation efficiency (CE), CD:guest molar ratio and increase in bulk formulation by using an UV–visible competitive method, phase solubility studies as well as ¹H and DOSY ¹H NMR titration experiments. For the first time, a new algorithmic treatment that combines the chemical shifts and diffusion coefficients variations for all guest protons was applied to calculate K_f. The position of the hydroxy group in carvacrol and thymol did not affect the stoichiometry of the inclusion complexes but led to a different binding stability with CDs. 2D ROESY NMR experiments were also performed to prove the encapsulation and illustrate the stable 3D conformation of the inclusion complexes. The structural investigation was accomplished with molecular modeling studies. Finally, the radical scavenging activity of carvacrol and thymol was evaluated by the ABTS radical scavenging assay. An improvement of this activity was observed upon encapsulation. Taken together, these results evidence that the encapsulation in CDs could be valuable for applications of carvacrol and thymol in food.

pHEMA hydrogels with pendant triazinyl-β-cyclodextrin as an efficient and recyclable reservoir for loading and release of plant-based mosquito repellents: a new aqueous mosquito repellent formulation

Poly(2-hydroxyethyl methacrylate) hydrogels with pendant β-cyclodextrins as an efficient and recyclable reservoir for loading and release of plant-based mosquito repellents.